Allow down migration of tsv to happen

Otherwise, the following step will always fail:

https://github.com/opf/openproject/blob/dev/docs/operations/migrating/packaged/postgresql-migration.md#index-attachments-for-fulltext-search

db/migrate/20180122135443_add_tsv_columns_to_attachments.rb

@@ -29,7 +29,7 @@
 #++
 
 class AddTsvColumnsToAttachments < ActiveRecord::Migration[5.0]
-  def change
+  def up
     if OpenProject::Database.allows_tsv?
       add_column :attachments, :fulltext_tsv, :tsvector
       add_column :attachments, :file_tsv, :tsvector
@@ -37,11 +37,25 @@ class AddTsvColumnsToAttachments < ActiveRecord::Migration[5.0]
       add_index :attachments, :fulltext_tsv, using: "gin"
       add_index :attachments, :file_tsv, using: "gin"
     else
-      reversible do |dir|
-        dir.up do
-          warn "Your installation does not support full-text search features. Better use PostgreSQL in version 9.5 or higher."
-        end
-      end
+      warn "Your installation does not support full-text search features. Better use PostgreSQL in version 9.5 or higher."
+    end
+  end
+
+  def down
+    if column_exists? :attachments, :fulltext_tsv
+      remove_column :attachments, :fulltext_tsv, :tsvector
+    end
+
+    if column_exists? :attachments, :file_tsv
+      remove_column :attachments, :file_tsv, :tsvector
+    end
+
+    if index_exists? :attachments, :fulltext_tsv
+      remove_index :attachments, :fulltext_tsv
+    end
+
+    if index_exists? :attachments, :file_tsv
+      remove_index :attachments, :file_tsv
     end
   end
 end